1. Field of the Invention
The invention relates to an electronic circuit for measuring AC electrical energy, and more particularly to an improved circuit using a current transducer which provides an analog signal that is responsive to the time derivative of the metered circuit current and a pulse width modulation circuit which produces a pulse width signal having a duty cycle that is proportional to the metered circuit current.
2. Description of the Prior Art
It is known in the art to provide electronic circuits for measuring AC electrical energy. One type of circuit uses a multiplier circuit to produce a signal which is responsive to the product of the current and voltage on the metered circuit, and a voltage-to-frequency converter to produce an output that is responsive to this product signal.
Various methods are known for producing the product signal. For example, U.S. Pat. No. 4,242,634 discloses an electronic meter using a variable-transconductance multiplier circuit. U.S. Pat. No. 4,254,376 discloses a circuit employing a pulse-width, pulse-height multiplier. A circuit employing a pulse width signal modulated with the metered circuit voltage is used to control a plurality of analog switches connected to a signal proportional to metered circuit current in U.S. Pat. No. 4,315,212. A related method is used in U.S. Pat. No. 4,182,983. In the patent, a pulse width signal is modulated with a quantity that is responsive to the metered circuit current. The pulse width modulated signal is used to control analog switches that are supplied with a quantity which is responsive to metered circuit voltage in order to produce a signal that is proportional to the power flowing through the metered circuit. The current responsive signal is produced by an integrator circuit connected to the output of a mutual conductance current transducer. This transducer provides an output signal which is proportional to the time derivative of the metered circuit current.
The apparatus described in the aforementioned U.S. Pat. No. 4,182,983 provides excellent performance over a variety of operating conditions. It would be desirable, however, to provide a circuit having improved temperature stability, noise immunity, and freedom from device offset voltage effects.